The applicant will continue to describe the nature and topography of the several receptor sites presented at the cell surface by systems mediating membrane transport. We seek to describe the response these structures make when occupied by a suitable substrate and cosubstrate, by using substrates with restricted or increased potentialities for producing these responses or for undergoing other metabolic changes. We are trying to identify the forces producing uphill transport by using substrates that make exaggerated respones to gradients across or within membranes; also to describe amino-acid-dependent ATPase activity associated with cellular membranes. Amino acids with dissociable side chains (e.g. glutamic acid, certain diamino acids) are used to explore how these are accommodated by transport systems ordinarily serving for neutral amino acids, and to see what proton movements result. Changes in substrate structure will be used to localize structures responding to these changes at the receptor site. The cooperation between parallel transport systems will be studied to see how they produce the observed steady-state distribution of metabolites. BIBLIOGRAPHIC REFERENCES: Christensen, H.N. (1975) Recognition Sites for Material Transport and Information Transfer, in current topics in membranes and transport, Vol. 6, pp. 227-258, p. Bronner and A. Kleinzeller, eds., Acad. Press, N.Y. CHRISTENSEN, H.N. and Hanlogten, M.E. (1975) A Cycle of Deprotonation and Reprotonation Energizing Transport, Proc. Nat. Acad. Sci. US, 72, 23-27.